Chapter 25, HSP for Pharma and Cosmetic applications
· Knowing the HSP of a pharmaceutical or cosmetic ingredient allows you rationally to think through its general bio-compatibility with DNA, skin etc.
· Formulating ingredients for controlled compatibility or, if required, incompatibility, is a rational process. The evaporation modeller in the Optimizer can help you understand how a solvent blend might change during evaporation – keeping critical components in solution or, if you wish, ensuring that some components fall out quickly and, for example, concentrate at the surface.
· The analysis of skin permeability based on Jmax instead of permeability coefficient seems to offer a lot of insight into practical skin formulation issues – in particular making the effects of permeation enhancers fall into a rational process of thinking about solvency rather than the restrictive and confused terminology such as “lipophilic v hydrophilic”.
· The Diffusion modeller can be used for many different issues. Permeability through skin is one example, permeability out of microcapsules is another. The HSP distance is crucial for understanding the solubility part of the Jmax = Solubility * Diffusion Coefficient/Thickness equation, and the diffusion coefficient itself can be strongly concentration dependent, another reason for knowing the HSP distance. From knowledge of the HSP distance it is possible to predict whether a microcapsule is likely to show Zero Order diffusion (a large distance) or Fickian diffusion (a smaller distance). Which behaviour is desired depends on the application, but it can be built-in rationally.
As an aid to rational chemical, excipient or solvent use we have included a number of .hsd tables in HSPiP. The first is a list of HSP for all the GRAS (Generally Regarded As Safe) where we could provide a meaningful value. Here’s a snapshot from the start of the list within GRAS.hsd:
Figure 1‑1 Some GRAS chemicals
A typical use for the table would be to find a rational substitute for a chemical that is not on the GRAS list. For example, if your chemical were an acid with [16, 3, 8], then Caprylic Acid would be a good starting point for substitution.
Another important list in Pharma is the Q3C(R3) list with Type 1 and Type 2 “bad” solvents and Type 3 and Type 4 “good” solvents. Here we’ve made it particularly easy to work out a rational substitution of Types 1 or 2 with Types 3 or 4. We’ve provided Q3C(R3) 1 and 2.hsd which you can load into the main form. This lets you view the key properties of these bad solvents. For example, if you wanted to get rid of chloroform from a formulation, you have the properties ready for the next phase:
Figure 1‑2 Using Q3C(R3) to replace a “bad” solvent
By highlighting Chloroform, then going to Solvent Optimizer into which you have loaded the Type 3 and 4 solvents as Q3C(R3) 3 and 4.sof you already have your target correctly set. Clicking on the “2” button gives you Cumene/Ethanol as a good HSP match. Of course there is more to matching than just the HSP. The Optimizer lets you rationally match for other properties such as Relative Evaporation Rate etc.
We’ve done something even more. Thanks to a generous HSPiP user we have a large list of EU cosmetic/food approved chemicals. Via their CAS numbers Hiroshi was able to generate SMILES for a large number of the chemicals (many others are ill-defined mixtures). He then used Y-MB to create the Food and Cosmetics.hsd and .sof files. In searches for alternatives in formulations, this is a most helpful. Load Food and Cosmetics.sof into the Optimizer and you will find a rich source of alternatives. Of course many of these aren’t solvents so you have to use caution. But we think it’s important to give you such a large database to explore this rich domain of “approved” chemicals. Please note, of course, that “approval” can change and many of these chemicals come with restrictions. We provide it as a courtesy to our users, not as an infallible guide to food and cosmetics best practice.
Although this is a tiny chapter, we think it’s a pointer to things to come. Solubility and permeability concepts in Pharma and Cosmetics have, in our opinion, been dominated for too long by one-dimensional concepts such as LogP, LogD and LogS. How much richer the discourse becomes when scientists can discuss issues in three dimensions rather than one!